Electric vehicles, hybrid vehicles, fuel cell vehicles, and the like have recently been developed and put into practical use as environmentally friendly vehicles. Such vehicles have mounted thereon a motor and a power supply apparatus for driving the motor.
Mounting a plurality of batteries on such vehicles is under consideration for the purpose of increasing an allowed running distance only by means of the batteries.
Japanese Patent Laying-Open No. 2002-10502 discloses a charge and discharge device for batteries that can charge and discharge a plurality of batteries simultaneously.
For electric vehicles, it is desirable that an allowed running distance on a single charge is long. For hybrid vehicles having mounted thereon an internal combustion engine, a battery and a motor, it is also desirable that the allowed running distance on a single charge without employing the internal combustion engine is long, when the hybrid vehicles are configured such that the battery is rechargeable from an external source.
To increase the allowed running distance on a single charge, the amount of energy of batteries mounted on the vehicle needs to be increased. Increase in the amount of energy can be achieved by: 1) increasing the energy capacity per battery cell; or 2) increasing the number of battery cells mounted.
With the above technique 1), it is difficult to ensure a desired amount of energy due to the presence of the upper limit of strength of a case for a battery cell. The technique 2) may be achieved by increasing the number of cells in series or in parallel.
Increasing the number of cells in series results in a high voltage. However, increasing the number of cells in series is restricted by the upper limit of withstand voltages of an inverter, a motor and the like which are electric loads. In contrast, connecting battery cells in parallel, without the provision of a power adjustment device, will deteriorate some of the batteries, although the required capacity can be ensured. This means that the capability of the mounted batteries cannot be fully utilized.
Further, vehicles having a wheel-driving motor mounted thereon are usually equipped with an auxiliary battery for driving an auxiliary load, in addition to a high-voltage battery of several hundred volts for driving the motor.
Charging the auxiliary battery and supplying power to the auxiliary load may be carried out at a voltage obtained by down-converting the voltage at the high-voltage battery by a DC/DC converter.
In this case, however, mounting a plurality of high-voltage batteries for increasing the allowed running distance will require some measures to be taken for balanced charging of the plurality of high-voltage batteries because one of the high-voltage batteries is required to supply power to the auxiliary load as well.